A Zn-based plated steel sheet in which a steel sheet is plated with Zn or a Zn alloy has excellent corrosion resistance, strength, workability, and the like, and also shows an aesthetically pleasing appearance. For these reasons, it is widely used for automobiles, housing, home appliances, and the like. In order to perform arc welding of Zn-based plated steel sheets, heat is applied while a welding wire is supplied between the Zn-based plated steel sheets which are materials to be welded. Thereby, they are joined. Consequently, the Zn-based plated steel sheets as materials to be welded are exposed to heat generated by electric arc during arc welding of them. Then, Zn vapor may be generated during welding because the boiling point of Zn (906° C.) in the plating layers is lower than that of Fe in the steel sheets. The vapor may enter into a welding section when it is in a molten state, and may be trapped after it is solidified to create cavities (blowholes) inside the welding section. Further, the blowholes may form openings (pits) when they grow to reach the surface of the welding section. Particularly in the case of lap fillet arc welding, Zn vapor generated from the overlaid portion of Zn-based plated steel sheets may tend to enter into a melted section, and ascend toward the surface of the melted section to form blowholes and pits inside the welding section. (Hereafter, the term “blowhole” encompasses the term “pit”.)
Various methods have been proposed to reduce development of blowholes. Provision of a space (gap) between welding members is effective. For example, Patent Document 1 proposes a method in which a gap of about 0.5 mm is provided between overlaid members to be welded, thereby allowing a generated gas to escape to the opposite side of a welding section (see the left lower column on page 1). Further, Patent Document 2 proposes a method as a conventional example in which protruded portions are provided on at least either one of two base materials to form a gap around a welding section, thereby allowing a vaporized low-boiling point material to diffuse and escape to the outside through the gap (see Paragraph 0005). These methods are effective to reduce development of blowholes. Nonetheless, it is difficult to sufficiently reduce development of blowholes throughout the entire length of a weld bead. In particular, the cooling rate of a weld metal is larger in a region formed after the start of welding (starting end portion) and a region formed before the end of welding (terminal end portion) as compared with a central region formed between them, and thus development of blowholes is difficult to be reduced. In view of the above, improvements have been demanded.
Patent Document 1: Japanese Unexamined Patent Application, Publication No. H07-246465
Patent Document 2: Japanese Unexamined Patent Application, Publication No. S62-179869